The present invention relates to resistive thin films in cathode ray tubes for arc suppression.
Arcing in cathode ray tubes used in color television is not a new phenomenon. Arcing occurs in the electron gun area of the cathode ray tube and causes damage to both the electron gun and the electronic circuitry which is responsible for the operation of the gun. The problem has become potentially more serious because of the trend towards the use of higher operating potentials (up to 30 kv) to enhance the brightness of the picture. There are several mechanisms by which arcs may occur and cause voltage/current fluctuations which are responsible for electron gun damage. Examples of these mechanisms include field emission in the G3-G4 region of the tube neck and conductive particle contamination. It is known to coat the neck portion of the cathode ray tube with a resistive thin film to reduce field emission. It is also known to deposit a highly conductive graphite film on the tube funnel. However, if the film deposited in either of these regions does not have adequate scratch resistance and adhesion characteristics, particles of this film may break loose and contaminate the tube, thereby causing the arcing problem previously referred to. Loose particle contamination arises from the frictional effect of the snubber contacts connected to the G4 electrode being in contact with the graphite film in the funnel region. Further, contamination also occurs merely from normal manufacturing procedures and from normal use.
One known tube having such a resistive thin film on both the neck and funnel of the tube is described in U.S. Pat. No. 3,355,617 to Schwartz et al. The film on the neck region is formed by applying a liquid coating of Fe(NO.sub.3).sub.3.9H.sub.2 O and Mn(NO.sub.3).sub.2 (51% sol.) and H.sub.2 O. The coating is then baked to drive off the water and decompose the nitrates, yielding a film essentially of oxides of iron and manganese and having an electrical resistance in the range of 10.sup.9 to 10.sup.12 ohms per square. The tube funnel is coated with colloidal graphite to produce a highly conductive film. However, films produced in such a manner could never be made less resistive without additional components. A uniform mixture which could be used for both neck and funnel areas is desirable. Also the salts mentioned in the Schwartz patent are in an aqueous solution. In order to bake out such a tube and to be sure no residual water remains, two firing steps are necessary which is highly uneconomical.
To enhance the adherence characteristic of the film there are several known film compositions which include a binder. A typical binder containing film is that described in U.S. Pat. No. 3,791,546 to Maley. The coating formulation of this patent includes a liquid solution of Fe.sub.2 O.sub.3 particles, graphite particles and a sodium silicate (Na.sub.2 O:SiO.sub.2) binder. However, the use of a binder complicates the film forming procedure. Also, while the binder may enhance the adherence characteristic of the film, it may degrade the scratch resistance characteristic. Lastly, silicates are hydroscopic and bind H.sub.2 O which poisons cathodes.